Oxide semiconductors are semiconductor materials having higher electron mobility than silicon-based materials such as amorphous silicon (a-Si). For example, it is considered that use of oxide semiconductors for channel layers of thin film transistors (TFT) can produce highly credible TFTs with low leakage current when no voltage is applied. TFTs including oxide semiconductors have thus been developed.
Quaternary oxide semiconductors (hereinafter also referred to as IGZO semiconductor) containing indium (In), gallium (Ga), zinc (Zn), and oxygen (O) as constituent atoms are considered more suitable for TFT than other oxide semiconductors because they have not only high mobility but also such characteristics as mentioned below.
First, IGZO semiconductors can be formed into a film at relatively low temperatures around room temperatures to about 150° C. In the case of using the aforementioned silicon materials, a film of TFT is formed at high temperatures of not lower than 300° C. Therefore, a TFT cannot be directly formed on a base material which is inappropriate for a high temperature atmosphere, such as flexible substrates including a film base material. However, use of an oxide semiconductor makes it possible to form TFT directly on a flexible substrate. Moreover, an IGZO semiconductor can be formed into a film by a sputtering system and thus can be produced by simple procedures.
Furthermore, thin films formed of an IGZO semiconductor can transmit visible light and are thus excellent in transparency. For this reason, such films can be applied for use in transparent electrodes including indium tin oxide (ITO) or the like.
Meanwhile, characteristics of IGZO semiconductors change depending on the composition of constituent atoms. Patent Document 1 discloses a phase diagram showing preferable compositions of constituent atoms of oxide semiconductors which are suitable as channel layers of TFTs. Patent Document 2 discloses preferable compositions of constituent atoms of oxide semiconductors which are suitable as transparent electrodes, such as TFT.